Noise limiting circuit



G. W. FYLER NOISE LIMITING CIRCUIT Filed June 14, -1941 Inventr George'v His Attorney oc't. 6, 1942.

SMM/ Patented Oct. 6, 1942 NOISE LIMITING CIRCUIT George kW. Fyler,Stratford, Conn., assignor to General Electric Company, ajcorporation ofNew York Y Application June 14, 1941, Serial No. 398,088

(Cl. Z50- 20) 15 Claims.

My invention relates to a noise limiting circuit particularly adaptedfor use in radio receiving apparatus.

This application is a continuation-impart of my application Serial No.364,160, filed November 4, 1940, and assigned to the same assignee asthe present invention.

It is an object of my invention to provide an improved noise limitingcircuit which effectively discriminates between undesired noise impulsesand desired signals impressed on the input of radio receiving apparatusand which prevents such impulses from appearing in the output circuitsof the apparatus. Y Y Y Another object of my invention is to provide animproved noise limiting circuit for preventing noise transients fromappearing in the receiver output whenever these transients exceed athreshold level, which is automatically determined by the carrier levelof the received signal as well as its modulation level. Y

Another object of my invention is to provide an improved noise limitingcircuit of this general type which is equally effective in suppressingnoise impulses received along with weak signals or with relativelystrong signals.

Still another object of my invention is to provide an improved noiselimiting circuit which automatically adjusts itself under all signalVconditions to provide highly eiiectivelimiting action with a minimum ofdistortion ofr the desired signal.

Still another object of my invention'is to provide an improved noiselimiting circuit which effectively silences the receiving' apparatus inrespense to a transient which exceeds a predeter-Y mined limiting level,this level being automatically controlled by the received signalsthemselves, and which provides rapid recovery of the receiving circuitsto normal operation following such an impulse, under all conditions ofoperation.

It is still a further object of my invention to provide an improvednoise limiting circuit which is simple, economical and Vreadilyincorporated in existing receiving circuits.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however,together with further objects Aand advantages thereofmay best beunderstood by reference to the following description taken in connectionwiththe accompanying drawing, in which Fig. 1 diagrammaticallyrepresents the circuits of a conventionalized form, which embodiesmyinvention; and Figs, 2 through 4 are graphs'which will be referred tofor a better understanding of the operation of my invention.

In the superhetercdyne receiving apparatus illustrated in Fig. l, thesignals received at the antenna I9 are amplified and converted intomodulated carrier waves of intermediate high frequency in the radiofrequency amplifier and converter II. I 'he construction andv operationof these circuits are well known to those skilled in the art and hencethey are indicated only t schematically.

superheterodyne type of radio receiver, partly in 5'5'I The intermediateYfrequency signals `are next amplified. Two stages I2 and I3 ofintermediate frequency amplification are illustrated, though of coursethere may be more or less than this number. For reasons that willshortly become apparent,l it is desirableto vvgive theseintermediatefrequency amplifiers a relatively wide band characteristic.VThis may be accomplished by various means familiar to thoseskillcd inthe art. Thus, inthe illustrated embodiment, the coupling transformersILL-I5 and IBare diagrammatically represented-as being adjusted to thedesired frequency by means ofmovable cores.V Where the intermediatefrequency is relatively high the windings of these transformers may betuned by their distributed capacity alone, as is indicated in thedrawing by the dotted capacities connected across the respectivewindings.

Operating potentials for the amplifiers `I2 and I3 are supplied from asuitable power supply source, represented conventionally by the battery2G. The anodes of these amplifiers are connected tothe positive terminalof the source 2i). Their screen grids are connected to a point 2| on ableeder resistor I'I shunted across this source. The cathode of theamplifier I2 is also returned to ground through a portion I9 of asecondbleeder resistor I-8,AI9, the resistor I9 being variable formanual volume control purposes.

The amplified carrier waves of intermediate frequency, modulated by theaudio signals, are impressed upon the second detector circuit 22 fromthe secondarywinding 23 of the last intermediate frequency transformerIB. The signal detector 2li is of the diode type having an anode 25 anda cathode 26. The signal detector circuit eXtends from the anode 25,through the secondary winding 23 of the input transformer I6, a diodeload resistor :21 and an intermediate frequency by-pass capacitor 28 inparallel, to the cathode 26, which is grounded.

The operation of the second detector circuit 22 is well known to thoseskilled in the art and will not be detailed here. The detector 24 iseffective to demodulate the waves impressed upon the transformer |6. Thedemodulation products developed across the diode load resistor 21include audio frequency components and a direct current component. Theaudio frequency components are coupled to the audio and poweramplifiers, indicated schematically by the block 29, through a couplingcapacitor 30. The amplified audio frequency currents appearing in theoutput of the amplifier 29 are supplied to any suitable translatingdevice, such as the loud 4 speaker 3|. y A

As is well known, interfering electrical disturbances of short durationand considerable magnitude are often received on the signal channelalong with the desired signal. These may form an undesirable noisebackground with the desired signal, or even mask it completely. Suchdisturbances are generally grouped under the term, impulse noise," asdistinguished from hiss or thermal agitation noise. These disturbancesmay arise from various causes; for example, within this category arenatural atmospheric static surges, man-made electrical disturbances, asfrom high frequency apparatus, ignition systems and the like, and othersharp impulses. The characteristics of these undesired noise pulses mayvary widely, depending upon their source, butin all cases they have adeleterious effect upon the fidelity of signal reproduction either bydirect interference With the signals or through shock excitation effectsin the receiving system.

In order to limit the effect of such noise impulses, a noise suppressionnetwork is provided. As is disclosed in my aforesaid application SerialNo. 364,160, this network includes a second diode detector 40 reverselyconnected in parallel to the signal diode 24 through a capacitor 4|. Adischarge circuit, comprising resistors 42 and 43 in series, is alsoconnected across the capacitor 4|. For reasons that will shortly becomemore fully apparent, the time constant of the network 4|, 42, 43, i. e.,the product of the capacity and resistance of this network expressed inseconds, is made relatively long as compared to the time constant of thesignal diode load network 21, 28. In other words, the time constant ofthe limiter network is made long as compared to the period of the lowestmodulation signal frequency to be supplied to the amplifiers 29. Theinternal resistance of the diode 4|) in the current-conducting directionshould also be approximately equal to the internal resistance of thesignal diode 24 for best results.

Considering for the moment only those elements of the noise limitingnetwork just described, it will be observed that the diode 40 is incircuit with the secondary winding 23 and the network 21, 28 and that ispoled to pass current through these elements, when conductive, in theopposite direction from current flowing in the signal diode circuit 22.This circuit extends from the anode 44 of the diode 40 through thenetwork 4|, 42, 43, the network 21, 28, and the secondary winding 23 ofthe transformer I6 to the cathde 45.

The signal diode 24 passes current on the peaks of those half cycles ofthe applied carrier wave which makes its anode positive, and themodulation voltages appearing on the network 21, 28 reproduce themodulation envelope. On the peaks of alternate half cycles, the maximumvoltage impressed across the diode 40 from the second detector 24 isapproximately equal to the sum of the maximum diode load voltage on thenetwork 21, 28, and the peak voltage of the applied carrier Waves. Thesetwo voltages are approximately equal and hence the peak voltages appliedacross the diode 40 and network 4|, 42, 43 are approximately equal totwice the peak amplitude of the carrier waves developed across thewinding 23.

Due to the relatively long time constant of the network 4|, 42, 43, thediode 40 functions as a peak rectifier on the outward peaks of audiomodulation and charges the capacitor 4| up ac- Y cording to the peakvalues of the signal and modulation levels. The diode 40 therefore drawsfrom the signal detector circuit 22 only the small current necessary toreplenish the charge on the capacitor 4| as it leaks o through theresistors 42 and 43. The potential on capacitor 4| varies with themodulation envelope so that noise peaks can be limited substantially atthe peak signal level, as will shortly appear. At zero modulation, orvery low levels of modulation, limiting takes place approximately at thecarrier level. The bias potential on diode 40 developed from peakrectication of the voltages across signal diode 24, thus may varybetween approximately two and four times the unidirectional component ofthe carrier voltage developed on diode load resistor 21.

Assume now that an undesired noise impulse of greater magnitude than thereceived signal and of short duration is impressed on the secondary A 23of the transformer I6. A low impedance path for voltages of eitherpolarity is now provided through one or the other of the two diodes,since the diodes are reversely connected to pass current through theoutput load resistor 21 in both di- Y, rections and capacitor 4| is oflow impedance for impulses which are conducted by diode 40. Therefore,since no rectification of the noise transient takes place, it iseffectively suppressed, producing substantially no effect upon the audio`output circuits 29. l

It may be noted at this point that the effectiveness of the noisesuppression circuits is materially increased by employing wide bandintermediate frequency circuits preceding the detector and limiter andrelatively narrow band audio circuits following them. This fact becomesapparent from a study of impulse excitation characteristics of tunedcircuits. In a wide band circuit the train of oscillations set up by asingle short sharp impulse, has a very high decrement; whereas, in anarrow band circuit, the converse is true. In other words, the length ofthe decay train is inversely proportional to the acceptance band widthof the system. On the other hand, it has been found that the peakamplitude of the train is almost directly proportional to the bandwidth. Therefore, it can be shown that the average value of a givenimpulse train is practically independent of band width.

From the above principles, it follows that sharp noise impulsesimpressed on the wide band intermediate frequency amplifiers I2 and I3remain sharpand of short duration in transmission through thesecircuits. If these impulses are now limited substantially to the peakcarrier level in the manner previously described, very little energyremains. Then, if they are further filtered at the diode load 21, 28 andfurther reatively narrow band audio circuits 29, they are actuallyreduced in amplitude far below the signal level.

It has been pointed out in my aforesaid application Serial No, 364,160that the eiectiveness of a noise suppression circuit of this generaltype, operating in the manner previously described, is greater on strongsignals than on weak signals. The reason for this will become more fullyapparent froma consideration of the curves of iI jig. 2 in conjunctionwith the following descripion.

The upper curve in Fig. 2 represents the recovery characteristics of thenoise limiting circuit under weak signal conditions. For illustration,it is assumed that a sharp noise transient exceeding a constant signallevel 50 suddenly charges the capacitor 4l up to twice its previousvalue, as indicated by the peak 5I. The condenser 4| now discharges backtoward the signal level through the resistors 42 and 43 along thelogarithmic curve determined by the time constant RC of the network. Itwill be seen that the actual recovery time is approximately representedby the time interval ti.

The lower curve in Fig. 2 represents therecovery characteristic of th'esame circuit when the noise transient exceeds the signal level by thesame amount as before but where the signal level 52 is many timesgreater. The condenser now discharges from the peak 53 toward the signallevel along a curve of the general form shown. Since the time constantRC is the same, it will be apparent that the time interval t2 ismaterially shorter than t1.

The time required for the capacitor 4| to discharge back to the peaksignal level determines both how much of the useful signal is suppressedand how quickly the noise limiting circuit is restored to normal. Aquick recovery rate is particularly important on sustained impulse noisewhich tends to maintain capacitor 4l continuously charged, impairing thelimiting action Consequently, it follows that the voltage on thecapacitor 4l, derived from peak rectioation of the signal and carriervoltages, is a measure of the limiting effectiveness in the general formof circuit under consideration. Considered another way, the limitingaction varies with the condenser discharge current in the resistors 42and 43. It will be recalled that the capacitor voltage, and hence thiscurrent, varies both with' the intensity of the received carrier signaland with its percent modulation. The discharge current will varygenerally within limits, as indicated by the cross-hatched area betweenthe curves 54 and 55 of Fig. 4, representing conditions of 100 per centmodulation and zero per cent modulation, respectively. l

summarizing, in the simplified form of limiter circuit considered up tothis point, the noise limiting action is least eiective on weak signalswhen it is most needed to preserve the intelligibility of the receivedsignal. In order to alleviate these conditions, it has been shown in myapplication Serial No. 364,160 that improved limiting action on weaksignals may be obtained by the addition of a source of potential inseries with the discharge circuit for the capacitor 4l. This source ispoled to produce a small current flow, under no-signal conditions,through the noise diode 40. It is also pointed out in my aforesaidapplication that the addition of such a potential source increases thespeed of recovery of the noise suppression circuit after a noiseimpulse, particularly for weak signal reception, since it assists indischarging the capacitor 4I. However, itis also pointed out that thisimproved limiting action is secured at some sacrice of delity. Harmonicdistortion in the desired signals isincreased due to the additional loadplaced onthe signal detector circuit. While such distortion can betolerated in voice-communication work and in weak signal reception,where intelligibility is the primary objective, it may becomeobjectionable when operating on strong signals.

Ideal noise limiting would be much' more closely approached if the noiselimiting circuits were fully automatic, so as to limit substantially tothe peak signal level, and yet had a constant rate of recovery under allsignal conditions. In accordance with my present invention, this isaccomplished by applying a variable bias potential to the condenserdischarge network and providing means for automatically varying thisbias potential in response to the capacitor voltage so as to maintainthe discharge current in the network substantially constant under allsignal conditions.

In accordance with th'e present invention, a control circuit including agrid-controlled amplifying device 69 is incorporated in the noiselimiting circuit. This device is represented in Fig. l as a triodehaving an anode 6|, a grid 62 and a cathode 63. The grid 62 is connectedto the junction point 64 between the capacitor 4l, resistor 42 and anode44. The cathode 63 is connected to the junction point 65 between theresistors 42 and 43. The anode 6I is connected t0 the lower end of theresistor 43 through a source of anode operating potential, representedby the battery 66. The anode voltage for the amplier 6i) may be adjustedby means of a movable tap 6l. The grounded terminal of the capacitor 4Iis connected to an intermediate point on the potential source 66 bymeans 0i a movable tap 68.

It will be seen that the resistor 42 is included in the grid circuit ofamplier 6i) and the resistor 43 and potential source 66 in its anodecircuit. For reasons that will shortly be apparent, the resistor 42 isof very high resistance as compared to the diode load resistor 2l;whereas the resistor 43 is of the same order of magnitude as resistor21. For example, in one particular embodiment of my invention, a valueof 10 to 20 megohms for the resistor 42 and about 50,000 ohms for theresistor 43 were found to be satisfactory.

The operation and adjustment of the noise limiting and control circuitswill be better understood :from a consideration of the curves of Fig. 3in conjunction with the following description. The manner in which thevarious voltages, indicated at E1, E2, E3 and E4 inl'ig. l, vary withthe received signal intensity is represented by the several curves ofFig. .3 bearing corresponding symbols.

First assume that the grid 62 is at ground potential, that no signalsare being received and that anode potential has just been applied to thecontrol device 66. The device 6! now draws anode current through theresistor 43, producing the voltage drop E2, which is of opposite sign tothe adjustable bias voltageEa between the lower end of resistor 43 andground. Consequently, a net voltage equal to their algebraic sum isimpressed between point 65 and ground. This voltage E24-E3 is of suchvalue that the cathode 63 has a positive potential with respect toground under no-signal'conditions. This in turn causes a small currentto flow through resistor 42, noise diode 40, transformer winding 23, andthe diode load resistor 2l to ground. The grid 62 is biasedapproximately to "cut-off by the voltage drop across resistor 42 and isheld substantially at zero potential with respect to ground. Thus, acondition of equilibrium is reached practically instantaneously betweenthe anode current in the amplifier 60 and the grid bias developed acrossthe resistor 42.

Assume now that a signal is impressed upon the receiving apparatus.Signal voltages are now developed upon the signal detector 24. Thecapacitor 4l charges through the noise diode 40, due to the peakrectification action of the noise limiting circuit, as previouslydescribed. The voltage E4 across the capacitor 4l now increases withincreased carrier or modulation in a sense to render the point 64 morenegative. As a result, the anode current in the control amplifier 60tends to decrease, thereby causing the voltage E2 to decrease. A newcondition of equilibrium is now immediately established. Within theloperating range of the control amplier 60, its grid potential changesonly negligibly. No grid current flows under any conditions.

It will thus be apparent from the foregoing and from an inspection ofFig. 3 that the voltage E24-E3 at the cathode 63 instantaneously followsf the voltage E4 at the grid 62 so as to maintain a substantiallyconstant difference between them (the voltage E1) over a wide range ofsignal intensities. The voltage E1 will remain substantially constantfor all values of signal and noise impulse intensities up to the pointwhere the magnitude of E4 approximately equals the magnitude of E24-E3under no-signal conditions. The value of E3 is adjusted to give thenecessary range of operation for the noise limiting circuit under alloperating conditions of the receiver. It may be zero under someconditions.

The value of E1 is determined by the voltage` applied to the anode 6Iand is approximately equal to the anode voltage divided by theamplification factor of the tube 60. It may be adjusted by moving thetap 61.

It will also be observed that the Voltage E4 at the point 64 can neverbecome positive with respect t ground because resistor 42 is of veryhigh resistance as compared to resistor 21 and diode 40 becomesconducting whenever its anode 44 is rendered positive.

If E1 has a constant value, as shown in Fig. 3, then the dischargecurrent through the resistors 42 and 43 will also be substantiallyconstant, since resistor 42 is very much larger than resistor 43, aspreviously described. The curve 'l in Fig. 4 represents the dischargecurrent under these conditions. It will be observed that it issubstantially constant over a wide range of signal intensities and thatit is independent of the per cent modulation of the received carrierwave.

The rate of recovery for the RC network 4I, 42, 43 is now substantiallyconstant under all signal conditions. Therefore, as the signal levelincreases, the ratio of discharge current drawn by the limiter networkto the signal current owing in the signal detector circuit 22 decreases,reduce ing harmonic distortion in the audio signal to an extremely lowlevel. At the same time, the limiting action is highly effective underweak signal conditions.

Merely for the purposes of illustration, the follil lowing data is givenfor a particular radio receiv- 75 ing apparatus embodying my invention.These values were found to give satisfactory results in a particularcase although they are not to be regarded as necessarily applicable toall embodiments of my invention.

Band Width of I. F. amplifiers About 300 k. c. Band Width of audioamplifiers About 5-10 k. c. Capacitor 4| .01-.1 mfd. Resistor 42 10-20megohms. Resistor 43 About 50,000 ohms. Voltage E3 30 volts.

Control amplifier 66: Type 6J5, anode voltage on amplier 150-300 volts.

A type 6J5 amplifier was used for the control tube in this particularapplication because it is a general purpose tube of suitableamplification factor. With one adjustment of this particular apparatus,the voltage drop E2 upon the resistor 43 under no-signal conditions wasabout 46 volts. Hence, the cathode 63 was at about plus 16 volts aboveground and the grid 62 at about ground potential. Under strong signal ornoise conditions the grid 62 and the cathode 63 decreased together so asto maintain substantially 16 volts bias on the grid 62 at all timesuntil the cathode potential reached a maximum of about minus 30 volts,corresponding to minus 46 volts on the grid 62 with respect to ground.

It will of course be apparent to those skilled in the art that manymodifications within the scope of my invention may be made. For example,in some cases it may be desirable to include an additional capacitor,connected between the cathode 63 and ground, of a suitable size toassist in the recovery of the noise limiting circuit after a very strongnoise transient. Such a capacitor will permit the voltake E1, andtherefore the discharge current, to increase momentarily in proportionto the charge effect of a noise transient, but only during the recoverytime of the noise limiting network.

Other modifications will doubtless occur to those skilled in the art.Hence, While I have shown a particular embodiment of my invention, itwill be understood that I do not wish to be limited thereto, but Icontemplate by the appended claims to cover any modifications as fallwithin the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a radio receiving system, a signal channel normally operative forthe translation of signal voltages, signal-responsive means forestablishing a threshold limiting level controlled by said voltages andfor disabling said channel in response to a transient noiseimpulseexceeding said level, said means tending to restore said channeltoward operative condition following said impulse at a time ratedependent upon the instantaneous magnitude of said level, and meansresponsive to the instantaneous magnitude of saidthreshold lever formaintaining said time rate substantially constant irrespective of saidlevel.

2. In a radio receiving system, a signal channel normally operative forthe translation of signal voltages, means for charging a capacitor to athreshold limiting level determined by said voltages, means fordisabling said channel in response to a transient noise impulse whichincreases the voltage on said capacitor above said level, meanscomprising aA discharge circuit for discharging said capacitor back tosaid levelv following said impulse, thereby to restore said channel tooperative condition, and means responsive to the voltage on saidcapacitor for maintaining a substantially constant current in saiddischarge circuit over a wide range of voltage variations on saidcapacitor.

3. In a modulated wave receiver for operation on carrier waves modulatedover a band of signal frequencies and subject to interfering noisetransients, the combination comprising a signal detection circuitincluding an input impedance, a signal rectifier and an output resistorconnected in series, said resistor being bypassed for carrierfrequencies, a noise suppression diode having an anode and cathodereversely connected across said signal rectifier through a capacitanceelement, a resistance element connected in shunt to said capacitanceelement through a source of variable bias potentials, said elementshaving a relatively long time constant and said source being poled tobias said anode positively in the absence of said carrier waves, andmeans responsive to voltages on said capacitance element for varyingsaid bias potentials.

4. In combination with a source of potentials subject to undersiredtransients, a capacitance, means responsive to said potentials forcharging said capacitance, means for discharging said( said capacitance,thereby to maintain a substantially constant discharge current from saidcapacitance over said range.

5. In combination with a source of signalmodulated high frequencycarrier potentials subject to interfering noise impulses, a signal de-`cuit serially comprising a second unilaterally conf ducting dischargedevice having an anode Yand a cathode, said output impedance and acapacitor, said devices being poled to pass current through said outputimpedance in opposite directions when conductive, means coupled to saidsource for impressing said carrier potentials on both of said circuits,means comprising said second device and capacitor for effecting peakrectification of the carrier and signal potentials impressed on saidsecond device, thereby to bias said anode negatively with respect tosaid cathode, said second device having a low impedance for noiseimpulses which overcome said bias, whereby said impulses tend to chargesaid capacitor through said second device, means for discharging saidcapacitor comprising a resistance in circuit therewith, and meansresponsive to the voltages on said capacitor for maintaining asubstantially constant voltage across said resistance over apredetermined range of voltage Variations on said capacitor.

6. In a system for translating signal potentials subject to undesirednoise transients, a capacitance, means responsive to said potentials forcharging said capacitance, means for discharging said capacitancecomprising a resistance connected in circuit therewith, means forimpressing a bias potential across said capacitance and resistance in apolarity to assist in discharging said capacitor, and means responsiveto the voltages onv said' capacitance for varying said bias potential.

7. In a systemfor translating carrier waves modulated by a band ofsignal frequency waves and subject to undesired noise transients, acapacitance element, means responsive to said waves for charging saidelement in one polarity, means for discharging said element comprising aresistance element in circuit therewith, said elements having a timeconstant long as compared to the period of the lowest signal frequency,means for impressing a bias potential across said elements in a polarityto assist in discharging said capacitance element and to maintain acurrent flow through said resistance element and charging means, andmeans responsive to the voltages on said capacitance element for varyingsaid bias potential so as to maintain said current flow substantiallyconstant over a wide range of voltages on said capacitance element.

8. In combination with means for receiving signal-modulated potentialssubject to interfering noise transients, a signal detection circuitcoupled to said means including a rst unilaterally conducting dischargedevice, a noise suppression circuit coupled to said means seriallyincluding "a second, reversely connected, unilaterally conductingdischarge device and a capacitance, a discharge circuit for saidcapacitance comprising a resistance and a source of variable biasvoltage, said source being poled to maintain a current iiow through saidsecond device and resistance, and means responsive to voltages developedacross said capacitance through rectification of potentials impressed onsaid noise suppression circuit for varying said bias voltage so as tomaintain a substantially constant current flow through said resistanceover a Wide range of voltage variations on said capacitance. v

9. In combination with means for receiving carrier waves modulated by aband of signal frequency waves and subject to undesired transients, asignal detection circuit coupled to said means serially including asignal diode and an output load impedance, a noise detection circuitcomprising a noise diode reversely connected across said signal diodethrough a capacitancev element, a discharge circuit for said elementcomprising a resistance element in series with a source of variable biasvoltage, said source being poled to maintain a current iiow through saidresistance element and noise diode, said elements having a time constantlong as compared to the period of the lowest signal frequency, wherebysaid noise diode develops potentials across said elements by peakdetection of the waves appearing across said signal diode, and meansresponsive to said potentials for varying said bias to maintain saidcurrent iiow substantially constant over a wide range of potentialvariations on said capacitance element.

l0. In a modulated wave receiver for operation on carrier wavesmodulated over a band of signal frequencies and subject to interferingnoise transients, the combination comprising a signal detection circuitincluding an input impedance, a signal rectifier and an output resistorconnected in series, said resistor being bypassed for carrierfrequencies, a noise suppression diode having an anode and cathodereversely connected across said signal diode through a capacitanceelement, a resistance element connected in shunt Ato said capacitanceelement through means supplying variable bias potentials, said elementshaving a relatively long time constant and said bias potential meansbeing poled to bias said anode positively in the absence of said carrierwaves, and means responsive to voltages on said capacitor for varyingsaid bias potentials.

11. In combination with a source of signal potentials subject toundesired transients, a capacitance, means responsive to said potentialsfor charging said capacitance, means for discharging said capacitancecomprising a resistance in circuit therewith, means for impressing biaspotentials on said resistance to maintain a predetermined currenttherethrough in absence of said signal potentials, said means includingan impedance in the anode circuit of a grid-controlled amplifyingdevice, and means responsive to the voltages on said capacitance tomaintain said current substantially constant over a wide range ofvoltage variations on said capacitance, said last means comprisingconnections for impressing voltages on said resistance upon the grid ofsaid device.

12. In apparatus for receiving carrier waves modulated by signalfrequencies and subject to interfering noise transients, a signaldetection circuit serially comprising an input impedance, a signal diodeand an output impedance, a noise suppression diode reversely connectedacross said signal diode through a capacitor, a discharge networkconnected in circuit with said capacitor comprising a resistance havingtwo sections, a thermionic amplier having grid and anode circuits,saidgrid circuit including one section of said resistance and said anodecircuit including a source of anode operating potential and the othersection of said resistance, said anode circuit being connected in suchpolarity that ow of anode current produces a bias potential across saidother section tending to maintain said noise diode conductive.

13. In apparatus for receiving carrier waves modulated by signalfrequencies and subject to interfering noise transients, a signaldetection circuit serially comprising an input impedance, a signal diodeand an output impedance network of suitable time constant for detectionof said signals, a noise suppression diode reversely connected acrosssaid signal diode through a capacitor, a discharge network connectedacross said capacitor comprising first and second resistances in series,the time constant of said capacitor and discharge network being longrelative to that of said output impedance network, a thermionic devicehaving an anode, a cathode and a grid,

said grid being conductively connected to the outer extremity of saidrst resistance, said cathode being conductively connected to thejunction of said resistances and said anode being conductively connectedto the outer extremity of said second resistance through a source ofanode operating potential, the direction of anode current ow throughsaid device and said second resistance being such that the voltage dropthereby produced across said second resistance tends to maintain saidnoise diode conductive.

14. In apparatus for receiving carrier Waves modulated by signalfrequencies and subject to interfering noise transients, a signaldetection circuit serially comprising an input impedance, a signal diodeand an output impedance network of suitable time constant for detectionof said signals, a noise suppression diode reversely connected acrosssaid signal diode through a capacitance, a discharge network connectedacross said capacitance comprising a resistance and a source of biaspotential, said potential tending to render said noise diodenon-conductive, said resistance having relatively high value and saidresistance and capacitance having a relatively long time constant ascompared to said output impedance network, a thermionic device havinggrid and anode circuits, said grid circuit including a major portion ofsaid resistance and said anode circuit including a source of operatingpotential and the remaining portion of said resistance, said anodecircuit being so connected that iiow of anode current through saidremaining portion produces a voltage drop thereacross opposing said biaspotential, and means for adjusting said potentials to maintain apredetermined current ow through said resistance and noise diode.

15. In a radio receiving system, a signal channel normally operative forthe translation of signal waves, an energy storage device, means forestablishing a threshold limiting potential level across said devicecontrolled by said waves and varying as a function thereof, means forincreasing the voltage across said device momentarily in response to atransient noise impulse exceeding said level and for disabling saidchannel so long as said voltage exceeds said level, and means responsiveto the instantaneous magnitude of said threshold level for causing saidvoltage to decrease upon cessation of said impulse toward said level ata substantially constant time rate irrespective of the value of saidlevel, thereby to restore said channel to operative condition.

GEORGE W. FYLER.

